1. Introduction
This invention relates to radiation sensitive compositions comprising a radiation sensitive component and a resin binder blend. In particular, the resin blend is a mixture of novolak resins which can provide highly uniform and thick coating layers.
2. Discussion of Related Art
Radiation sensitive compositions include photoresist compositions which are well known in the art and described in numerous publications including DeForest, Photoresist, Materials and Processes, McGraw-Hill Book Company, New York, 1975. Photoresists comprise coatings produced from solution or applied as a dry film which, when exposed to radiation of an activating wavelength, are chemically altered in their solubility to certain solvents. Photoresists can be negative-acting or positive-acting. Negative-acting resists are initially soluble in its developer, but following exposure to activating radiation, become insoluble in developer, thereby defining a latent image. Positive-acting resists work in the opposite fashion, radiation exposure making the resist soluble in developer.
Positive-working resists in general comprise a radiation sensitive compound and a film forming polymer blend. The radiation sensitive compounds, or sensitizers as they are often called, most frequently used are esters and amides formed from o-quinone diazide sulfonic and carboxylic acids. These esters and amides are well known in the art and are described in DeForest, suora, pp. 47-55, incorporated herein by reference.
The resin binders often used with the o-quinone diazides in commercial practice are the alkali soluble phenol formaldehyde resins known as the novolak resins. Photoresists using such polymers are reported in U.K. Patent No. 1,110,017, incorporated herein by reference. These materials are the product of reaction of a phenol with formaldehyde under conditions whereby a thermoplastic polymer is formed.
Positive photoresists using novolak resins as binders are often used as masks to protect substrates from chemical etching in photo engraving processes. For example, in a conventional process for the manufacture of printed circuit boards, a copper-clad substrate is coated with a layer of a positive-working photoresist, exposed to actinic radiation to form a latent circuit image in the photoresist coating, developed with a liquid developer to form a relief image and etched with a chemical etchant whereby unwanted copper is removed and copper protected by the photoresist mask is left behind in a circuit pattern. For the manufacture of printed circuit boards, the photoresist must possess chemical resistance, must adhere to the circuit board substrate, and for high density circuits, must be capable of fine-line image resolution.
Similar photoresists are also used in the fabrication of semiconductors. As in the manufacture of printed circuits, the photoresist is coated onto the surface of a semiconductor wafer and then imaged and developed. Following development, the wafer is typically etched with an etchant whereby the portions of the wafer bared by the development of the photoresist are dissolved while the portions of the wafer coated with the photoresist are protected, thereby defining a circuit pattern. For use in the manufacture of a semiconductor, the photoresist must possess resistance to chemical etchants, must adhere to the surface of the semiconductor wafer and must be capable of fine line image resolution.
For a number of processes, application of a thick photoresist film can be highly desirable. A coating film having a thickness of about 3.0 to 3.5 microns or greater is generally recognized and is referred to herein as a "thick" coating film. For an additive metallization process, a developed thick photoresist layer can be necessary. For example, it is often necessary to plate up a thickness of 5 microns or greater. To accomplish such plating, a developed photoresist image having well resolved sidewalls of a height equal to or greater than the thickness of plating is typically required.
Thick photoresist coatings also can be required to provide a uniform coating layer over topography. More specifically, in microelectronic applications, it is often necessary to apply a uniform coating layer over chip contact holes. To provide sufficient uniformity of the resist coating layer, application of a thick resist layer can be required.
Use of a metal plasma etch can also necessitate use of thick photoresist coatings. A metal plasma etch is typically quite aggressive to a photoresist film. By employing a thick resist film, a portion of the resist coating can be sacrificed during the plasma etch process without destroying the patterned image.
While the foregoing makes clear that photoresists that can be applied as thick films have significant utility, many known photoresists do not provide a highly uniform thick coating layer. Such lack of uniformity can be unacceptable for many applications.
It thus would be desirable to have a radiation sensitive composition that could be applied as a thick and uniform coating layer.